The on-resistance of a vertical-form power MOSFET is strongly dependent on an electrical resistance of a conduction layer (drift layer) portion. The doping concentration that determines the electrical resistance of the drift layer cannot exceed a limit set according to a breakdown voltage of a pn junction formed by a base layer and the drift layer. There is, therefore, a trade-off relationship between the device breakdown voltage and the on-resistance. In low-power electrical devices, it is important to improve this trade-off. The trade-off has a limit determined by device materials, and the way to realizing a device with a lower on-resistance than existing power devices lies in exceeding this limit.
As an example of a MOSFET that solves this problem, a structure called a super junction (SJ) structure is known in which p-pillar layers and n-pillar layers are embedded periodically in the drift layer. In the SJ structure, charge quantities (quantities of impurities) included in the p-pillar layer and n-pillar layer are set to be the same, to create a quasi non-doped layer and maintain a high breakdown voltage. Furthermore, a low on-resistance below the material limit is realized by passing a current through the highly doped n-pillar layer. In addition, an even lower on-resistance can be realized by increasing the impurity concentration in the p-pillar layer and n-pillar layer while narrowing a horizontal-direction period of the SJ structure.
In the SJ structure, however, in addition to the pn junction formed from the base layer and the drift layer, a pn junction is formed between the p-pillar layer and n-pillar layer in the drift layer. Consequently, an area of the pn junctions expands and a drain-source capacitance Cds increases. Hence, the rate of change of drain voltage (dV/dt), which is dependent on Cds and the gate-drain capacitance Cgd and usually controlled by a displacement current flowing in Cgd, becomes difficult to suppress within a predetermined range. The result is a problem of increased switching noise. To solve this problem, there is demand for a power semiconductor device capable of reducing switching noise by maintaining a low on-resistance through use of an SJ structure.